Mitchum or Blackburn for the Mullard 6922?

Do Mitchum production and Blackburn production sound different in the Mullard 6922? I believe I once read somewhere that Mitchum made tubes for the military, whereas Blackbutn did not, so, presumably, with better materials and/or tighter tolerances or testing. 
Codes when I have it in hand.
petg60... Mullard e88cc code is B5H3.
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Branded IEC Mullard?
B5H3 is only the manufacturing date and plant, should have more digits for type of tube before those.
Are these numbers etched or same lettering as Mullard?
I have seen such tubes on eBay and they are Mullard made ones but not sure if rebranded or not.